Transcription in simple terms is the synthesis of RNA from DNA with the help of RNA polymerase. Prokaryotic transcription is less complicated than the eukaryotic one. This is also contributed by the factor that prokaryotic organisms have a rather simple structure. The nucleus is not defined ,lacking membrane bound organelles and histone proteins. Prokaryotic genetic material contains of bacterial chromosome as well as plasmids that carry additional genetic information that can be transferred by bacteria through sex pili.
While you write an essay on transcription in Prokaryotes make sure that you cover these important points.
- List the different steps in prokaryotic transcription
- Discuss the role of promoter
- Describe how and when transcription is terminated
Prokaryotes including bacteria and archaebacteria, lack membrane bound nuclei and organelles, the bacterial chromosome is different from an eukaryotic one , it is a covalently closed structure that lacks histone proteins. Transcription meaning the synthesis of RNA from DNA template with the help of bacterial RNA polymerase. Although the procedure occurs both in prokaryote and eukaryotes, if differs with the RNA polymerase, promoters and the transcription factors involved. In prokaryotes the transcription procedure involves the partial unwinding of the DNA helix . The region that unwinds forms a bubble like structure that is known as the transcription bubble.
The nucleotide pair on the DNA helix that corresponds to the first 5 mRNA nucleotide is called the +1 site or the initiation site.The nucleotide after the initiation site are numbered with + numbering . These are known as downstream nucleotides.
Transcription involves the following steps
In prokaryotes transcription and translation occurs simultaneously. Before we discuss about the transcription process we need to understand two important factors that assist in transcription.
- RNA polymerase.
Prokaryotic RNA polymerase is composed of two α,β, β′,ω and σ subunits. These subunits together are known as the holoenzyme. Whereas α,α,β, β′,ω constitute the core enzyme.
α subunits are necessary to assemble the polymerase on the DNA.
β subunit binds to the ribonucleoside triphosphate
β′ subunit binds to the DNA template.
The sigma (σ ) subunit is the most important of all as it plays its role in the initiation of transcription. It gives transcriptional specificity so that the polymerase can identify and synthesize mRNA from the initiation site only if the sigma factor binds to it. So obviously the absence or error in sigma factor leads to abortive transcription.
RNA polymerase doesn’t require a primer.
Prokaryotes transcription is concerned with two promoters one at -10 and the other at -35 upstream of the initiation site. The -10 upstream promoter is also known as Pribnow box( named after the scientist).The AT rich -10 region also facilitates in the unwinding of the DNA template . The promoter regions are recognized by the sigma factor and help in initiating the transcription process.
The -10 consensus sequence is TATAAT
The -35 consensus sequence is TTGACA
The transcription elongation begins with the release of the sigma subunit from the polymerase. This dissociation of teh sigma factor allows he core enzyme to proceed along the DNA template synthesizing mRNA in the 5′ to 3′ direction at the rate of 40 nucleotides/sec.
- The core enzyme contains the catalytic site for polymerization probably with the β subunit.
- The first nucleotide in the RNA transcript is usually pppA or pppG.
- Synthesis occurs in the 5′ 3′ direction using 4 ribonuleotides (ATP,CTP,GTP,UTP)
- The 3′-OH at the end of the growing RNA chain attacks the α phosphate group of the incoming ribonucleotide to form 3’5′ phosphodiester bond.
- The complex of RNA polymerase , DNA template and New RNA transcript is called a ternary complex.
- The DNA template rewinds after the transcription bubble has passed.
- The RNA produced has the same sequence as the non template strand expect that the RNA contains uracil instead of thymine.
- Transcription continues until the termination is reached.
- The most termination signal is a GC rich region that is a palindrome followed by an AT rich region.
- The RNA made from the DNA palindrome forms an hairpin structure which is rich in G and C nucleotides and is followed by 4 or more U residues.
- Those termination sites that lack these hairpin structure take the help of a protein called rho. Rho protein helps recognize the termination site and stop transcription.
- Thus the RNA polymerase encounters a termination signal and ceases transcription, releasing the RNA transcript and dissociating from the DNA.